Cylinder key locks are conventionally used to secure many types of entrances. Their structure and operation are well-known and they are thus frequently subjected to tampering to gain unauthorized access.
The structure and operation of conventional cylinder key locks are described, for example, in U.S. Patent to Bauer et al. No. 3,494,158 and U.S. Patent to Murphy No. 3,541,822. In such locks, a rotatable inner cylinder or plug is housed in a larger fixed outer cylinder. In each of these cylinders, there are an in-line group of chambers which house upper and lower pins. The chambers in the two cylinders are disposed so that they are aligned when the lock is in the locked position. The inner cylinder has below the chambers a key slot. To operate the lock, a key is inserted through a restricted opening in the outer face of the inner cylinder, into this key slot. When the key is inserted, the upper edge of the key which is at various elevations raises the various pins in the aligned pin chambers. When the key is fully inserted, the pins are raised in their chambers such that the shear line between the upper and lower pins in each chamber is coincident with the meeting point of the aligned lower and upper chambers. Thus, with the key fully inserted, the inner cylinder may be rotated effecting the unlocking of the lock.
Such conventional cylinder key locks are susceptible to tampering and unauthorized opening. Two alternative techniques for such unauthorized opening are called picking and impressioning. Picking involves the use of a small tool which is inserted into the key slot and is then used to raise the upper and lower pins in the aligned chambers. As the pins are raised, a torsional force is imparted to the inner cylinder. When the pins reach the point at which the shear line is coincident with the boundary between the inner and outer cylinders, the inner cylinder will move slightly, or in some cases snap to a new position, because of the play in the lock. Under this condition, the upper pin may be mounted on the upper edge of the lower pin chamber on the inner cylinder. The process is repeated for each set of pins. When the process is complete, the lock may be opened.
Impressioning involves placing a key blank into the key slot and filing down the portion of the blank under each group of pins until the shear line is reached. The position of the shear line is determined much the same way as with picking. Each of these alternative techniques involves the raising or lowering of the pins until a point of lower resistance to the torsional force on the inner cylinder is reached and noticed by the tamperer.
A third technique for unauthorized opening which is effected in conjunction with picking the lock involves the rapid spinning of the inner cylinder with a spring-propelled insert.
Prior art cylinder key locks have incorporated features whose objective was resistance to tampering and unauthorized opening. Such prior art locks are disclosed in the previously mentioned Bauer et al. and Murphy patents and in U.S. Patent to Hines No. 3,349,588. These prior art devices use traps or recesses in the pin faces to resist one of the tampering techniques. None of the prior art techniques are capable of resisting all of the above described techniques.
It is therefore an object of this invention to provide a lock apparatus with features to resist a variety of tampering techniques. It is another object of this invention to trap the tampering tools inside the lock and to provide additional locking apparatus to prevent further tampering once the initial tampering has been attempted. It is a further object of this invention to resist not only the various tampering techniques individually but also when a tamperer attempts to use them in combination. Another object of the invention is to insure that the proper user of the lock is aware that tampering has been attempted.
Other objects of this invention will appear from the following description and the appended claims.